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IS leader Abu Bakr al-Baghdadi made his first appearance on video in Mosul in July 2014

One year ago, Islamic State announced the formation of its caliphate - a state governed in accordance with Islamic law, or Sharia, by God's deputy on Earth, or caliph. IS has been characterised by extreme violence, but thousands have still flocked to the territory it inhabits, and groups have pledged their allegiance.

Why has IS' caliphate attracted followers?

The last widely recognised caliphate - that of the Ottomans - was abolished 90 years ago. IS has tried to capitalise on the symbolism, romanticism and nostalgia attached to the notion of the caliphate for many Muslims across the world.

The caliphate, especially of early Islam (632-1258), enjoys an almost mythical status in Islamic literature and school curriculum in many Muslim countries.

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IS has showcased coins it has minted for its caliphate

The same goes for its related military conquests that allowed Muslim rule and religion to extend far beyond Arabia to include the Middle East, North Africa, large parts of Asia, and Spain. School literature is often filtered to leave out any negative aspects of caliphate rule, hence producing a glossy image of that institution.

The era was marked by scientific and cultural prosperity, with Muslims making important contributions to mankind.

Many young Muslims grow up reading, studying and hearing tales about the "golden age" of caliphate rule with a sense that it was the only era of Muslim history to be proud of and aspire to return to.

In his first ever public appearance shortly after the caliphate declaration, IS leader Abu Bakr al-Baghdadi urged Muslims across the world to migrate to IS territory.

He told them to "hold your heads up high for today you have a caliphate that will restore your dignity, power, rights and leadership". Soon, he promised, "Muslims will walk everywhere like masters".

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IS' magazine portrays a romantic version of its the caliphate while showing images of atrocities

Islamic State has exploited the appeal of the caliphate to justify its violent practices and punishments.

To mark the declaration, IS released the first edition of its glossy English-language magazine Dabiq. The title and theme of that issue was The Return of the Caliphate.

Since then, IS has tried to model itself on the caliphates of early Islam.

The group has tried to justify its harsh rule and use of cruel punishments against people living on its territory by alluding to earlier caliphate rule.

It has also mimicked caliphate terminology, for instance calling provinces "wilayat"; instead of governors, it appoints "walis"; and instead of ministries, IS has "diwan" - all administrative divisions and positions used under caliphate rule. These terms are presently obsolete in most Muslim countries.

Is the caliphate expanding?

IS has tried to model itself on early Islamic caliphates by "expanding" into wider territory, exploiting the positive Islamic literature on caliphate "conquests" of lands and peoples.

Beyond its major presence in Iraq and Syria, IS has made big public announcements about its presence in Egypt, Yemen, Libya, Saudi Arabia, Algeria and further into West Africa, Afghanistan and, most recently, in the Caucasus - inspiring or buying loyalty.

However, many of these "expansions" are exaggerated. Unlike the expansions of Islamic caliphates that involved Muslim armies marching to foreign lands, IS' expansion mainly comes from pledges of allegiance from existing militant groups.

Some have rebranded themselves as new IS "provinces", or wilayat, such as the Egyptian Ansar Bayt al-Maqdis and the Nigerian Boko Haram.

So is IS' caliphate getting stronger or weaker?

A crucial factor for the legitimacy of any caliphate - apart from wide Muslim acceptance and recognition - is to control sizeable territory and be able to provide protection and services to its constituents.

The appeal of IS' caliphate declaration for some was only made possible because it followed the militant group's swift capture of lands in Iraq and subsequently in Syria.

Its strong propaganda machine has made sure to highlight the group's "state-like" activities and services. These include healthcare, education, charity projects, and maintenance of electricity and roads - regardless of how superficial these services actually are.

Once IS starts losing significant territory and stops being able to provide protection and basic services to people living on its territory, it risks ceasing to be the state or caliphate it claims to be.

In recent months, IS lost important areas in both Iraq and Syria and has been embroiled in fighting against rival jihadist groups on various fronts.

There are also many reports of complaints from people living on IS territory who are suffering from poor living conditions and the harsh codes of conduct.

The group's leadership has recently made two appeals to Sunni Muslims who have fled in Iraq to return to IS-held territories, showing perhaps desperation for their support.

Is the caliphate here to stay?

IS has demonstrated an ability to bounce back from defeat and claim victories following setbacks.

After losing the northern city of Tikrit in Iraq in March, IS captured Baiji to the north and then Ramadi to the south. It has also recently made a brief, violent return into the Syrian border town of Kobane, from where it was repelled in January.

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An IS member destroys an icon of the Virgin Mary and Jesus on the wall of a church in Mosul, Iraq

While only a very small minority of Muslims, mostly youth, have been dazzled by IS' message, which is trumpeted by a strong propaganda machine, the numbers are significant enough to cause worry.

What is a caliphate?

An Islamic state ruled by a single political and religious leader, or Caliph

Caliphs are regarded by their followers as successors to the Prophet Muhammad and the leader of all Muslims

First caliphate came into being after Muhammad's death in 632

In the centuries which followed, caliphates had dominion in the Middle East and North Africa

The last widely accepted caliphate was abolished in 1924 by Turkish leader Kemal Ataturk after the collapse of the Ottoman Empire

The Ahmadiyya sect of Islam has recognised a caliphate for the last century, but it is only this group that does so

BBC Monitoring reports and analyses news from TV, radio, web and print media around the world. You can follow BBC Monitoring on Twitter and Facebook.